Network resources are commonly requested and retrieved using the Hypertext Transport Protocol (HTTP). A common approach to improving performance is caching network resources closer to the clients (often in several different data centers are differently geographically located). For example, content delivery networks (CDNs) are commonly used to cache resources such as static resources (e.g., resources that change infrequently such as images, videos, etc.) closer to clients.
Resources that frequently change (e.g., web sites for news, sports, etc.) may not be cached since the cached version may quickly become stale. Also, other types of resources may not be cached. As an example, HTTP includes a Cache-Control header field that is used to control the caching mechanisms along the request/response chain. The Cache-Control header field includes a “no-cache” directive that can be set by an origin server to specify that a cached version of the resource is not to be used to respond to a request without successful revalidation with the origin server.
To decrease the size of transmissions of resources, whether from a cache server or from an origin server, compression is typically used. For example, gzip is a common compression algorithm used to send compressed HTTP responses to clients.
Delta encoding in HTTP has been proposed in RFC 3229, “Delta encoding in HTTP”, January 2002, as a way to reduce the size of resource transmission between an origin server and the client. The delta encoding proposal described in RFC 3229 recognizes that if a web resource changes, the new version is often similar to the older version. RFC 3229 proposes that the difference between the versions be sent to the client (e.g., the browser on a client computing device such as a desktop computer, notebook, tablet, smartphone, etc.), and the client applies the delta to construct the new version. The approach described in RFC 3229 has not been widely adopted because in order to be effective for a resource that changes often, the origin server would have to store many versions of the website (e.g., an extreme would be a single version for each single client).
Another proposal for compression is the use of a shared dictionary compression over HTTP. In the shared dictionary compression, the client and server agree on a set of predefined elements that will be the same across the resources (referred to as cross-payload redundancy). For example, the header, footer, and/or other elements of the resource may be the same. With this proposal, the shared dictionary is downloaded to the client that contains strings that are likely to appear in subsequent HTTP responses. The server can then substitute those elements with a reference to the dictionary when sending the HTTP response and the client can use its dictionary to reconstruct the resource, thereby reducing the payload of the transmission. This approach has not been widely adopted in part because it is administratively difficult to maintain the dictionaries. For example, if a website is changed (e.g., it is redesigned), the references in the dictionary will likely need to be changed and disseminated to the clients. In addition, this approach requires support on the client devices.